1. Field of the Invention
The present invention relates to a print head for dot matrix printers and, more particularly, to a print head for dot matrix printers for impacting a printing stylus held by an armature toward the paper placed over the outer periphery of a platen by the swift movement of the armature energized and driven and electromagnet.
2. Description of the Prior Art
Dot matrix printers for forming letters and symbols such as numerals on recording paper by the dot matrix are well known. Since the shapes of all kinds of letters and symbols are synthesized as desired, dot matrix printers have recently been used in various fields as printers of information processing apparatus. Such a dot matrix printer has a plurality of styli, and printing operation is carried out by the impact operation of the end portions of the styli. The impact operation of the stylus is carried out by attracting or releasing an armature fixed at one end of the stylus at a predetermined timing by an electromagnet. As the structure of the styli and the driving portions of the armatures of such a print head are conventionally known the type of the system in which the impact operation is carried out by attracting the armature such as those disclosed in Japanese Patent Laid-Open Nos. 64459/1986 and 33463/1988, and U.S. Pat. No. 4,230,038, and the type of the system in which the impact operation is carried out by releasing the armature such as those disclosed in Japanese Utility Model Laid-Open Nos. 147/1985 and 3039/1985, and U.S. Pat. Nos. 4,634,301 and 4,652,158.
FIGS. 5(A) and 5(B) are a plan view and a side elevational view, respectively, of the stylus and the driving portion of the armature of the prior-art print head of the type of the system in which the impact operation is carried out by releasing the armature.
At one end of an armature 10, a stylus 12 is fixed. The other end of the armature is fixed to a print head holder 14 by a rod-like torsion spring 16. The torsion spring 16 is biased with a torsional force so that the armature 10 is driven to the impact operation when the armature 10 is released from the attraction of the electromagnet. The print head holder 14 is secured to the armature 10 by locating the end portion of the armature 10 between the branched portions 14a, 14b at the end of the print head holder 14 on the side of the armature 10, and piercing the branched portions 14a, 14b and the end portion of the armature 10 with the rod-like torsion spring 16.
The printing operation of the stylus 12 of such a dot matrix printer is carried out by releasing the armature 10 from the energizing means so as to drive the stylus 12 in the direction of impact. The impact operation of the armature 10 is performed with the rod-like torsion spring 16 as the fulcrum, and the driving force is produced by the spring return force of the torsion spring 16 which is provided in advance with torsion by the attraction of the electromagnet.
However, in the apparatus disclosed in Japanese Patent Laid-Open No. 64459/1986 having a system in which the impact operation is carried out by attraction of the armature, since the electromagnet on the armature is situated on the side of the stylus (on the impact operation side), the length of the stylus 12 inevitably becomes long because it is necessary for the stylus 12 to cross the electromagnet to reach the recording paper. In the apparatus disclosed in U.S. Pat. No. 4,230,038, although the electromagnet is situated on the opposite side of the armature relative to the stylus, a long wire is also used because it is necessary to bundle a plurality of styli at the center.
In the case of using such a long stylus, since the mass of the stylus is large, the printing speed is lowered and a stylus guide is required. The provision of a stylus guide involves a danger of deteriorating the printing performance due to the wear of the stylus guide.
According to the structure of the stylus and the driving portion of the armature shown in FIGS. 5(A) and 5(B) and used in the apparatus disclosed in Japanese Utility Model Laid-Open No. 147/1985 and U.S. Pat. No. 4,634,301, since the rod-like torsion spring 16 supporting the armature 10 at the end thereof is provided with a torsional force which warps the torsion spring 16 toward the direction of impact of the armature 10, it is necessary for securing the accurate impact operation to effectively prevent the bending caused by the torsion, namely, the warping of the torsion spring 16 in a direction deviated from the axial direction. Therefore, the rod-like torsion spring 16 is required both to secure sufficient rigidity for preventing bending and to secure the torsional spring property. Due to these contradictory requirements, the material and the configuration of the torsion spring are restricted.
In order to provide a torsional force for the torsion spring 16, it is necessary to firmly secure the torsion spring 16 to the branched portions 14a, 14b of the print head holder 14 and the armature 10 so as to prevent the positional deviation in the direction of rotation with respect to the armature 10. In this case, since it is necessary that the rod-like torsion spring 16 is narrow to a certain extent in order to secure the torsion force, if the torsion spring 16 is fixed by brazing, the brazing operation becomes complicated, resulting in the rise in cost. If the torsion spring 16 is fixed by screwing or press fitting, there is a fear of insufficient fixture. If the fixture of the torsion spring 16 is insufficient, the positional deviation in the rod-like torsion spring 16 is produced during printing, which disadvantageously results in the shortage of starting force of the armature, thereby making the impact operation impossible.
In this way, highly accurate and difficult operation is required for securing the rod-like torsion spring 16 to the armature 10, thereby inconveniently raising the cost.
Furthermore, since the fulcrum for driving the armature 10 (the position of the rod-like torsion spring 16) is located at the end of the armature 10, it is necessary to dispose the electromagnet between the end portion of the armature 10 and the stylus 12, so that if the energizing means serving as the means for driving the armature 10 is disposed on the opposite side of the armature 10 relative to the direction of impact (lower side in FIG. 5(B)), it is impossible to utilize the attraction of the energizing means for the impact operation of the armature 10 as it is.
It is therefore necessary to release the attraction of the energizing means for effecting the impact operation, and for this purpose, members such as a permanent magnet and a spring as well as the energizing means are required, thereby complicating the structure.